Common-mode feedback circuits stabilize the common mode voltage of differential outputs by adjusting the common mode bias currents. The two differential output voltages are averaged to form a common-mode voltage. The common-mode voltage is compared with a designated reference common-mode voltage. The difference is then amplified and converted into a common-mode output current to adjust the common-mode voltage. Most commonly used common mode feedback circuits fall into the following three categories: (a) Switched Capacitor CMFB, (b) Resistor averaged CMFB, (c) Differential Difference amplifier CMFB.
FIG. 1 illustrates a conventional circuit diagram of a switched capacitor common mode feedback (CMFB) circuit 100. The switched capacitor circuit 100 samples a common mode of outputs in one phase of the clock and controls common mode in another phase of the clock so there is no instantaneous control. A capacitor at the output reduces the effective bandwidth of an amplifier. A common mode control voltage is very sensitive to a parasitic which results in a common mode offset. Charge fed through from switches produces additional offset to the common mode. The effect of the charge fed through can be reduced by increasing the sampling capacitor but it increases output loading hence reduces bandwidth.
FIG. 2 illustrates a conventional circuit diagram of a resistor averaged common mode feedback (CMFB) circuit 200. The resistor averaged circuit 200 has advantages of instantaneous control of a common mode and minimum common mode offset because of no parasitic sensitivity and clock feed through. But a resistor at an output reduces gain by reducing output impedance. This problem is removed by buffering the outputs. But a buffer at the output reduces the output swing. So swing is lower than the switched capacitor CMFB 100 and buffers of outputs reduce a phase margin which leads to stability problems.
FIG. 3 illustrates a conventional circuit diagram of a differential difference amplifier common mode feedback (CMFB) circuit 300. The differential difference amplifier circuit 300 has an inherent buffering of outputs, so it has all the advantages of a resistor averaged circuit 200 (CMFB), except linearity. The circuit 300 has a worse linearity than both the circuits 100 and 200. But the linearity can be improved by increasing the channel lengths of input transistors and a gain of a common mode amplifier. But it has lower swing and requires a compensation to improve stability.
In another approach an operational amplifier having differential inputs and differential outputs with a predetermined common-mode output voltage independent of common-mode input voltage and an input voltage variation is provided. D.C. common-mode feedback is utilized to provide a differential amplifier having a precise common-mode output voltage, which is similar to the CMFB circuit 300 as illustrated in FIG. 3.
Therefore, there is a need for a novel continuous time common-mode feedback (CMFB) module that can provide a wider swing and a good linearity and which provides a wide bandwidth and a low systematic offset.